This invention relates to a metallic descaling process. More particularly, it relates to a process for controlling oxide scale formation and descaling thereof from metal articles.
In the process of manufacturing metal articles from the melt stage to a finished strip product such as flat rolled strip and sheet, bar, wire and tubular products, such processing frequently includes thermal treatments including heating or annealing in oxidizing furnace atmospheres to temperatures at which oxide scale readily forms on the article surface of various metal alloys including stainless steel alloys. To be used in conjunction with continuous annealing or heating processes, a scale removing bath must be of sufficient strength to remove the scale as rapidly as the strip emerges from the furnace. As the speed of the strip cannot be changed to accommodate the scale removal process and equipment, the scale removal process must quickly and efficiently condition and loosen the scale to facilitate removal. Generally, there are three different descaling processes which may be used: (1) a shot blasting plus an acid pickling; (2) a molten oxidizing salt or electrolytic scale conditioning followed by an acid pickling; and (3) a straight acid pickling process.
U.S. Pat. No. 3,043,758, issued July 10, 1962, discloses an electrolytic descaling and pickling process for stainless steel wherein the article to be descaled is subjected as an anode in an aqueous electrolyte of at least one neutral salt selected from a group consisting of chloride, sulfate and nitrate of an alkali metal including ammonium and thereafter dipping the article in a solution of mineral acid selected from the group consisting of sulfuric, nitric and hydrofluoric acid and mixtures of nitric and hydrofluoric acids. The electrolyte of the process is maintained at a pH of 1-7, and preferably 3.5-7 and nominally 5.5, and at a temperature ranging from 167.degree.-194.degree. F. (75.degree.-90.degree. C.) and subjected to an electric current for 10-60 seconds at a current density of 6-10 A/dm.sup.2 (0.38-0.64 A/in.sup.2).
Another electrolytic descaling process including a step of dilute acid immersion is disclosed in U.S. Pat. No. 3,254,011, issued May 31, 1966.
It is also known to use molten oxidizing salts to condition and loosen the scale and facilitate its removal. A typical process is disclosed in U.S. Pat. No. 3,260,619, issued July 12, 1966. Such molten salt baths generally require operating temperatures from 800.degree.-1000.degree. F. (426.7.degree. to 537.8.degree. C.) to condition and remove the scale and may be followed by a dilute acid pickling step. Although the high temperature salt baths are effective to loosen oxide scale, they also present numerous problems. Such processes require higher operating costs, and may result in strip distortion, punch marking, surface scratching and other chemical attacks of a strip or sheet. They also have the additional disadvantages of attacking the various components of equipment required to retain the salt and guide the strip through the bath, such as tank linings, rolls and the like, whick, in turn, can cause warping and distortion, especially of lighter gauge strip.
Attempts have also been made to minimize or eliminate the subsequent acid pickling bath from descaling processes. The use of an acid pickle step requires auxiliary equipment including exhaust systems, fume scrubbers, acid storage tanks and the like and also require elaborate programs for disposal of the acids. Thus, processes which eliminate or minimize the need for acid pickling are desirable. U.S. Pat. No. 4,012,299, issued Mar. 15, 1977, discloses such a descaling process requiring immersion into a first electrolyte, followed by a rinse and a subsequent immersion into a second electrolyte including at least one neutral salt such as sodium sulfate without the need of a subsequent acid pickling step. The patent also discloses that the second electrolyte has a pH ranging from 1-7 at a temperature of 120.degree.-200.degree. F. (48.9.degree. to 93.3.degree. C.) and be subjected to an electric current for more than four seconds having a current density greater than 0.1 A/in.sup.2 (1.55 A/dm.sup.2). Such a process has its disadvantages in that the first electrolyte is a molten oxidizing salt maintained from 400.degree. to 450.degree. F. (204.4.degree. to 232.2.degree. C.) and presents the numerous problems associated with such salts as described above.
Two other processes designed to eliminate the acid pickling step are disclosed in U.S. Pat. Nos. 4,026,777, issued May 31, 1977, and 4,066,521, issued Feb. 3, 1978. Both processes require a two-step process wherein the first step includes immersion in a bath of molten oxidizing salt. Such processes have the disadvantages associated with the higher operating temperatures of the molten oxidizing salt baths.
It has also been suggested in a U.S. patent application Ser. No. 238,896, filed Feb. 27, 1981, that a one-step high current density descaling process can be used to remove oxide scale with no pre- or post-treatment and no acid pickling. The process includes using a 15-25% sodium sulfate electrolyte maintained at a temperature of at least 150.degree. F. and passing an electric current through the body in the electrolyte at a current density of at least 3 A/in.sup.2 (46.5 A/dm.sup.2) for at least ten seconds and following with a water rinse. Such a high current density process, however, is particularly suited for small items such as tubing, wherein the anode lengths may be 4 to 6 feet and the total number of amperes required for a descaling process are relatively low. However, for strip mills, and particularly strip mills for wide strip, an anode may approach 40 feet in length. In such a descaling process, the total number of amperes used for descaling is far greater and the attending cost of such a high current density process may be practically prohibitive.
What is needed, therefore, is a descaling process which minimizes or reduces the need for an acid pickling step and all the problems and costs associated therewith. An electrolytic process should be a low current density process suitable for descaling continuous strip products. In meeting those objectives, it has been unexpectedly recognized that a process that controls the scale formation on the metal articles during manufacturing processes may provide a scale that is more easily removed. In conjunction therewith, a process which optimizes current descaling processes, such as a sodium sulfate electrolyte process followed by minimal acid concentration pickling, and which provides more efficient and less costly descaling in the manufacture of a more uniform product is also desirable.